As one of performances of an outer panel required for automobile parts such as doors, engine hoods, roofs or the like is dynamic panel stiffness. If the dynamic panel stiffness is lacking, so-called buckling sound by snap through is generated during car washing by applying a moving load to the outer panel with a palm, a cloth or the like along a form of the panel or during the wiping operation in waxing, whereby the quality feeling of the automobile is damaged significantly. Particularly in recent years, the buckling sound tends to be easily generated due to panel thinning associated with weight-saving of automobile body and diversification of panel design. Hence, securing dynamic panel stiffness becomes a significant issue with car manufacturers.
Heretofore, evaluation of dynamic panel stiffness of the outer panel is carried out as follows. That is, JP-A-559-009542 discloses a method wherein the outer panel is deformed by pushing an indenter onto a certain site of the outer panel, during which a relationship between load and displacement of indenter is converted into an electric signal that is recorded. In that method, the indenter can be separated from a measuring base to push the indenter onto the outer panel assembled into the automobile body and the load is measured by a load cell disposed on the indenter, and the displacement is calculated by integrating signal of an acceleration meter disposed on the indenter twice.
JP-A-S62-070730 discloses a method wherein an indenter of a panel stiffness measuring head integrally united with a load meter and a displacement meter is pushed onto a certain site of an outer panel transversely mounted onto the measuring base to deform the outer panel and a load is measured by the load meter. In that method, the indenter is pushed at the predetermined load to deform the outer panel to accurately perform the measurement of panel stiffness. The displacement meter is reset to zero at the deformed state and then the displacement quantity of the indenter is measured until completion of unloading while decreasing the load by retracting the indenter from the deformed state.
JU-A-H06-018947 discloses a method wherein panel stiffness of the outer panel in the automobile body is measured by attaching a pressing test unit to a robot arm. To accurately measure the snap through of the outer panel by that method, an indenter made from a subcolumnar aluminum material is attached to the pressing test unit through a load cell for the load measurement. The pressing test unit is moved to a front of a certain site of the outer panel and thereafter the indenter is pushed onto the outer panel with a hydraulic cylinder driven by a hydraulic hand pump to measure displacement of the indenter with a dial gauge.
JU-B-H07-014857 discloses a method of measuring panel stiffness of a roof panel for automobile body. In that method, a support member is supported by an arm horizontally extending above the automobile body to prevent a decrease in measuring accuracy due to misalignment between load meter and pressing shaft in the measuring operation, and the pressing shaft that applies a load to the roof panel is elevatably supported and a displacement meter for detecting a moving quantity of the pressing shaft is supported by the support member. Further, a slider provided with a load meter opposing to a rear end portion of the pressing shaft is elevatably supported and an air cylinder elevating the slider is supported by the support member, and the slider is allowed to fall by the air cylinder to move the pressing shaft downwardly through the load meter and the certain site of the roof panel is deformed by the pressing shaft to measure displacement of the pressing shaft with the displacement meter.
However, those conventional methods of measuring static panel stiffness evaluate load displacement of only the certain site of the outer panel pushed by the indenter so that it is impossible to evaluate the behavior of sequentially changing a deformation zone of the outer panel by moving the indenter to apply a moving load based on reproduction of a wiping operation of moving a palm, a cloth or the like along the surface of the panel while applying a constant load.
As to dynamic panel stiffness of the outer panel, a simple sensory evaluation has been hitherto performed in the manufacturing floor. In that evaluation, an inspector judges the presence or absence of generating the buckling sound by stroking the outer panel along its curved face with a palm, a cloth or the like while applying a load to determine an acceptance of the panel. Although the judgment of acceptance on the presence or absence of generating the buckling sound by such a simple sensory evaluation is sufficient in the manufacturing floor or quality assurance site, a quantitative evaluation way is required to study improvement of dynamic panel stiffness in the development stage. In the quantitative evaluation, acoustic data of the buckling sound can be analyzed to collect information such as loudness (sound pressure level), tone (frequency) and the like in addition to the presence or absence of generating the buckling sound, and it is possible to evaluate dynamic panel stiffness in detail. Also, to identify a decisive influence on dynamic panel stiffness, it is important to grasp deformation behavior of the outer panel. Further, to verify accuracy of simulation in performance prediction, it is necessary to compare the deformation state of the panel between analysis and measured result. To this end, it is desired to develop a technology of measuring dynamic panel stiffness in a higher accuracy.
It could, therefore, be helpful to provide a method and an apparatus for measuring dynamic panel stiffness of an outer panel for automobile parts.